1. Field of the Invention
The invention relates to a seat back frame of a vehicle seat, in which a headrest is able to be mounted to an upper portion of the seat back frame.
2. Description of Related Art
In a vehicle seat provided with a headrest for supporting a head of an occupant at an upper portion of a seat back, typically, a metal headrest mounting member is fixed to an upper portion of a seat back frame, and a headrest is attached to this headrest mounting member. More specifically, cylindrical headrest support made of resin is attached to this headrest mounting member, and the headrest is held to the seat back frame by a stay that extends down below the headrest being inserted into this headrest support. A seat back frame provided with such a headrest mounting member is described in Japanese Patent Application Publication No. 11-216037 (JP 11-216037 A) below, for example.
This seat back frame described in JP 11-216037 A includes an upper frame member that forms an upper side, and a reinforcing member provided parallel to and below the upper frame member. The headrest mounting member is fixed by being welded so as to cross over the upper frame member and the reinforcing member that are arranged one above the other. Having the headrest mounting member be fixed to the seat back frame in two locations, one upper and one lower, in this way makes it possible to ensure the rigidity necessary to support the head of the occupant with the headrest when a vehicle is collided with from the rear (hereinafter referred to as “when a collision occurs”), and is thus a measure against whiplash to the occupant.
Typically, when a vehicle is running or idling (hereinafter referred to as “during normal operation”), a floor panel resonates at a particular frequency (a resonance point) with an engine, and this vibration is also transmitted to the headrest via the seat frame that is fixed to the floor panel, regardless of whether an occupant is seated in the seat. According to the technology described above, rigidity for supporting the occupant is able to be ensured as described above by increasing the support rigidity of the headrest mounting member. However, the resonance frequency of the headrest (i.e., of the resonance with respect to the seat back frame) increases to close to the resonance point of the floor panel described above, so the headrest will end up amplifying the vibration level of the floor panel and vibrating, which is problematic.
Here, if the support rigidity of the headrest mounting member can be reduced, the vibration of the headrest can be reduced, but on the other hand, the rigidity for supporting the occupant when a collision occurs will also end up decreasing. That is, increasing the support performance for an occupant when a collision occurs and suppressing vibration of the headrest are technologically contradictive tasks as long as the support rigidity of the headrest mounting member is simply adjusted (i.e., increased and decreased). It is also possible to reduce the resonance frequency of the headrest by increasing the mass of the headrest, but this would result in a corresponding increase in both vehicle mass and cost, which is undesirable.